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Thermal battery with long life time and long shelf lifeRelated Patent Categories: Chemistry: Electrical Current Producing Apparatus, Product, And Process, Preserving Cell In Storage FeatureThermal battery with long life time and long shelf life description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20080096095, Thermal battery with long life time and long shelf life. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates to improved thermal batteries. More particularly the present invention relates to preventing storage self-discharge in thermal batteries. BACKGROUND [0002] Thermal batteries are thermally activated primary batteries comprising a series of cells consisting of a lithium-based anode, a cathode, an electrolyte between the electrodes and a pyrotechnic heat source. At ambient temperatures, the electrolyte is a solid, nonconducting salt. When power is required from the battery, an internal pyrotechnic heat source is ignited providing then enough heat to melt the solid electrolyte, thus allowing electricity to be generated electrochemically for periods from a few seconds to an hour. Employing solid state electrolytes in thermal batteries prevents storage self-discharge due to low ionic diffusion coefficient in solids. Thermal batteries have, therefore, a good shelf-life, require no maintenance, and can tolerate physical abuse (such as vibrations or shocks) during their storage life. Thermal batteries are most often used for military applications such as missiles, torpedoes, bombs, dispersed munitions, fuses, space missions and for emergency-power situations such as those in aircraft or submarines. [0003] Electrolytes used in thermal batteries usually melt at temperatures between about 350.degree. C. and about 450.degree. C., which require high amounts of pyrotechnic heating materials, and often cause a problem maintaining the high temperatures of the molten state. Melting the electrolyte in order to activate the battery requires significant periods of time from tens to hundreds of milliseconds. [0004] Generally, thermal batteries operate for seconds or for minutes, up to tens of minutes. Ideas for longer operation life were propose in the prior art: [0005] U.S. Pat. No. 5,770,329 incorporates the heat source in a cathode precursor wafer essentially Fe/KClO.sub.4, that upon ignition transforms into a FeO electrically conducting cathode. [0006] US 2003/0017382 introduces a superior all-lithium electrolyte, LiCl--LiBr--LiF eutectic composition with lowest melting temperature possible, as the lithium source that furnishes Li.sup.+ ions to the top surface of the cathode, where a good intercalation of Li.sup.+ ions and FeO takes place. Introducing all-lithium electrolytes as LiCl--LiBr--LiF, having higher melting and operation temperatures, instead of more conventional ones, i.e. LiCl--KCl, produces, however, a need for pyrotechnics and short discharge time due to early electrolyte solidification. [0007] The drawback of the batteries disclosed in both U.S. Pat. No. 5,770,329 and US 2003/0017382 is in high melting point of the electrolyte, i.e., higher than about 350.degree. C., resulting in relatively short operation times. [0008] The need for light weight thermal batteries operating at lower temperatures and having longer operation times together with long shelf-life led to several solutions proposed in the prior art: [0009] U.S. Pat. No. 4,117,207 offers an improved electrolyte of sodium tertrachloroaluminate with lower melting temperature in the range of 165.degree. C.-250.degree. C. However, due to the potential difference between the anode and cathode this low melting point of the electrolyte results in a substantial ion flow through the electrolyte and then in self-discharge of the battery. For this reason, battery shelf-life is considerably reduced. [0010] H1983 discloses the use of a supercooled liquid electrolyte that may remain liquid even below 0.degree. C. In the solid state, due to slow solid to solid reactions the electrolyte is highly resistive. Self-discharge for the same reasons mentioned above in relation to U.S. Pat. No. 4,117,207 takes place also in this type of battery. [0011] Storage potential difference between the electrodes, which may rise to a value of about 2.3 volts, may cause a substantial reduction in capacity during battery storage if self discharge occurs, thereby reducing the battery shelf-life, power generation output and operation time in an activated state. U.S. Pat. No. 5,900,331 aims at solving the problem of thermal battery storage self-discharge by introducing an insulating (epoxy/polymeric) layer between the pyrotechnic heat source, essentially a Fe/KClO.sub.4 homogenous mixture, and a steel cover positioned on top of the electrodes in a stack configuration, where the heat source is separated from the electrodes, and the cathode includes also the electrolyte. The temperature and 103 at 60.degree. C. than without the layer. [0012] The attempt to reduce storage self-discharge while introducing such highly electrically insulating polymeric demands adapting the process of manufacturing thermal batteries to include a further step of coating, which is evidently more time consuming and costly, and may cause short circuiting through the polymer layer. The apparatus thus offered, should also take into consideration stresses inflicted by mechanical vibrations or shocks, and therefore potential structural failure, and the strength of bonding between the epoxy insulating layer and the steel cover affected by physical properties as interfacial bond strength and difference in values of the coefficients of thermal expansion. [0013] Undesirable release of toxic residues to the surroundings as a result of the burning of the polymeric resin insulator in the process of activation, as well as potentially acidic by-products of that burning may cause corrosion and fast degradation of the battery metallic container as well as other corrosion susceptible parts. Also, release of gases will cause an increase of internal pressure of the thermal battery. This requires increase of battery case thickness. Moreover, the melting temperature of the electrolyte remains high, regardless of the addition of new resin layer, then the operating life time for the battery remains short. [0014] The present invention combines the benefits of long operation time upon ignition with practically zero storage self-discharge by using on one hand a low melt temperature electrolyte and on the other hand a unique configuration of the battery cell of two anodes having at most a minor electric potential difference between them. [0015] It is, therefore, an object of the present invention to provide a thermal battery with no storage self-discharge that overcomes the drawbacks of the prior art. [0016] It is another object of the present invention to provide a two-anode thermal battery comprising a heat-source, which is essentially part of the second anode and an electrolyte with lower working temperature. [0017] It is another object of the present invention to provide a thermal battery with long operation time upon ignition. [0018] It is still another object of the present invention to provide a thermal battery with an extended shelf-life. [0019] It is still another object of the present invention to provide a light-weight small volume thermal battery. [0020] It is still another object of the present invention to provide a thermal battery having a mechanically resistant configuration, and which does not emit gases inside the thermal battery or toxic residues to the surroundings upon ignition as well as corrosion inflicting products. [0021] It is still another object of the present invention to provide a method for eliminating storage self-discharge in a thermal battery. [0022] This and other goals and objects of the present invention will become apparent from the description and claims to follow. Continue reading about Thermal battery with long life time and long shelf life... Full patent description for Thermal battery with long life time and long shelf life Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Thermal battery with long life time and long shelf life patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. 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